Electronic banding compensation (ebc) of halftone-interaction banding using variable beam delays

ABSTRACT

Disclosed are methods and systems for compensating for process direction banding associated with a document processing system including a ROS. According to one exemplary embodiment, a ROS driver uses a plurality of beam delay/advance values to compensate for banding caused by an interaction of a halftone pattern and process direction density variations associated with the ROS.

REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of and claims priority to U.S.Utility patent application Ser. No. 14/091,748, filed Nov. 27, 2013 andentitled “ELECTRONIC BANDING COMPENSATION (EBC) OF HALFTONE-INTERACTIONBANDING USING VARIABLE BEAM DELAYS”, the entirety of which is herebyincorporated by reference.

CROSS REFERENCE TO RELATED PATENTS AND APPLICATIONS

U.S. patent application Ser. No. 13/313,533, filed Dec. 7, 2011, nowU.S. Publication No. 2013/0148172, published Jun. 13, 2013 by RobertHerloski et al. and entitled “Printing System, Raster Output Scanner,and Method with Electronic Banding Compensation Using Facet-DependentSmile Correction”; and,

U.S. patent application Ser. No. 13/334,251, filed Dec. 22, 2011 (nowU.S. Publication No. 2013/0163055, published Jun. 27, 2013) by RobertHerloski et al. and entitled “Process for Creating Facet-SpecificElectronic Banding Compensation Profiles for Raster Output Scanners”,are incorporated herein by reference in their entirety.

BACKGROUND

The present exemplary embodiments relate to printing systems with rasteroutput scanner (ROS) apparatuses and to techniques for mitigatingbanding errors. Reprographic printing systems are used to create markedimages on paper or other markable media, and improving the quality ofthe produced images is a continuing goal. Final image quality isaffected by various sources of noise and errors in a reprographicsystem, leading to density variations in the marking material fused tothe final print medium. In the reprographic process, a photoreceptortravels along a process direction, and images and text are formed asindividual scan lines or groups of scan lines (sometimes referred to asa swath) in a raster scanning process in a cross-process direction,where the process direction motion is much slower than the rasterscanning in the cross-process direction. Accordingly, the cross-processscanning direction is sometimes referred to as a “fast scan” direction,and the process direction is referred to as a “slow scan” direction.

Certain sources of reprographic system noise and errors cause periodicdensity variations in the process direction, which are sometimesreferred to as “banding” errors. Periodic density variations may becharacterized by frequency, amplitude, and phase in relation to afundamental frequency, as well as harmonics. Various sources of bandingexist in a marking (or print) engine. For example, raster outputscanners employ rotating polygon mirror apparatus driven by a motor,known as a motor polygon assembly or MPA, with one or more light sourcesbeing scanned by rotation of the MPA such that scan lines are generatedin the fast scan, i.e., cross-process, direction through reflection offa reflective facet of the rotating polygon mirror apparatus.

Differences in reflectivity, shape, profile, orientation, etc. indifferent reflective facets of the polygon lead to differences in imagedensity, i.e., color intensity, in the final print out which are afunction of which polygon facet was used to create a given scan line orswath of scan lines. As a result, the final print image may includebands of variations from the desired density that are periodic in theprocess direction. Other sources of banding errors include gears,pinions, and rollers in charging and development modules; jitter andwobble in imaging modules, as well as photoreceptors and associateddrive trains. Banding usually manifests itself as periodic densityvariations in halftones in the process direction. The period of thesedefects is related to the once around frequency of the banding source.If not addressed, such periodic process direction density variations canrender a reprographic printing system unacceptable, particularly wherethe banding errors are visually perceptible.

Banding can be addressed through reductions in the sources of such noiseor errors and/or by compensation in various reprographic systemcomponents in order to counteract its affects. In practice, it isdifficult to completely eliminate the error sources that contribute tobanding, or even to reduce them enough to avoid perceptible periodicdensity variations. In addition, customer requirements are continuallyreducing the amount banding that is deemed to be acceptable.Consequently, banding compensation techniques have become an importanttool in meeting reprographic system performance specifications. Forinstance, ROS exposure power can be varied in a controlled fashion tocompensate for banding, and conventional banding compensation techniquesinclude measurement of banding from one or more sources and the use ofthat information to actuate some correction strategy on a scanline byscanline basis to combat banding.

U.S. patent application Ser. No. 13/313,533, filed Dec. 7, 2011 byRobert Herloski et al. and entitled “Printing System, Raster OutputScanner, and Method with Electronic Banding Compensation UsingFacet-Dependent Smile Correction” and U.S. patent application Ser. No.13/334,251, filed Dec. 22, 2011 by Robert Herloski et al. and entitled“Process for Creating Facet-Specific Electronic Banding CompensationProfiles for Raster Output Scanners” describe electronic bandingcompensation for 1× and 2× ROS errors, related to the process directionbanding caused by revolution of a MPA (motor polygon assembly).

This disclosure provides methods and systems to compensate for bandingcaused by the interaction of a halftone design and swath to swathvariations associated with a ROS.

INCORPORATION BY REFERENCE

http://en.wikipedia.org/wiki/Phasor, 7 pages;

U.S. Patent Publication Application No. 2011/0298883, published Dec. 8,2011, by Ohyama and entitled “Image Forming Apparatus”;

U.S. Patent Publication Application No. 2011/0228030, published Sep. 22,2011, by Maeda and entitled “Image Forming Apparatus and Turn-On TimeCorrection Method;

U.S. Pat. No. 7,391,542, issued Jun. 24, 2008, by Tanimura et al. andentitled “Optical Scanning Apparatus and Image Forming Apparatus Usingthe Same”;

U.S. Pat. No. 6,636,253, issued Oct. 21, 2003, by Nihiguchi et al. andentitled “Light Scanning Apparatus”;

U.S. Pat. No. 6,307,584, issued Oct. 23, 2001, by Hirst et al. andentitled “Single Polygon Scanner for Multiple Laser Printer”;

U.S. Pat. No. 5,248,997, issued Sep. 28, 1993, by Summers and entitled“Facet Reflectance Correction in a Polygon Scanner”, are incorporatedherein by reference in their entirety.

BRIEF DESCRIPTION

In one embodiment of this disclosure, described is a raster outputscanner (ROS) for generating an image on a portion of a light-receivingmedium traveling along a process path past the ROS, the ROS comprising:a plurality of light sources operative to generate a plurality of lightoutputs; a driver operatively associated with the plurality of lightsources and operative to selectively actuate the plurality of lightsources individually according to corresponding scan line image data toproduce a corresponding plurality of modulated light outputs, the driveroperatively associated with a memory storing a plurality of beam timingvalues corresponding to each of the plurality of light sources, the beamtiming values set to compensate for banding caused by an interactionwith a halftone pattern and process direction density variationsassociated with the ROS, each beam timing value associated with across-process direction delay or advancement of a respective lightsource, the driver configured to selectively actuate the plurality oflight sources according to the corresponding beam timing values based onthe scan line image data to produce the plurality of modulated lightoutputs; and a scanner that directs the modulated light outputs from thelight sources toward a light receiving medium along the cross-processdirection to generate an image on at least a portion of the lightreceiving medium.

In another embodiment of this disclosure, described is documentprocessing system comprising: at least one marking device operative totransfer marking material onto an intermediate medium; a transferstation positioned proximate a travel path of the intermediate mediumand operative to transfer the marking material from the intermediatemedium to a printable media; and a raster output scanner (ROS)operatively associated with the at least one marking device to generatean image on a portion of a light-receiving medium, the ROS including: aplurality of light sources operative to generate a plurality of lightoutputs; a driver operatively associated with the plurality of lightsources and operative to selectively actuate the plurality of lightsources individually according to corresponding scan line image data toproduce a corresponding plurality of modulated light outputs, the driveroperatively associated with a memory storing a plurality of beam timingvalues corresponding to each of the plurality of light sources, the beamtiming values set to compensate for banding caused by an interactionwith a halftone pattern and process direction density variationsassociated with the ROS, each beam timing values associated with across-process direction delay or advancement of a respective lightsource, the driver configured to selectively actuate the plurality oflight sources according to the corresponding beam timing values based onthe scan line image data to produce the plurality of modulated lightoutputs; and a scanner that directs the modulated light outputs from thelight sources toward a light receiving medium along the cross-processdirection to generate an image on at least a portion of the lightreceiving medium.

In still another embodiment of this disclosure, described is a method ofmanufacturing or adjusting a document printing system, the methodcomprising: operatively coupling a raster output scanner (ROS) with amarking station in a document processing system; determining a pluralityof beam timing values corresponding to individual ones of a plurality oflight sources of the ROS so as to compensate for banding caused by aninteraction with a halftone pattern and process direction densityvariations associated with the document printing system; and storing theplurality of beam timing values in a memory associated with the ROS.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of a ROS process including a plurality of beamdelay values set to compensate for banding according to one exemplaryembodiment of this disclosure.

FIG. 2 is a plot of the dL* (image brightness variation) associated witha test ROS as the beam delay applied to a beam is varied.

FIG. 3 is a plot of the dL* associated with the test ROS used for FIG. 2as the intensity of a beam is varied.

FIG. 4 is a plot of the dL* associated with another test ROS as the beamdelay applied to a beam is varied.

FIG. 5 slows test results for a 20 μm beam delay phase sweep applied tothe test ROS fused for FIG. 2 and FIG. 3.

FIG. 6 is a simplified schematic system level diagram illustrating anexemplary multi-color document processing system in which the markingdevices include ROS operable using electronic banding compensationaccording to this disclosure.

FIGS. 7-9 are simplified schematic diagrams illustrating an exemplaryROS using an electronic banding compensation process including beamdelays according to various aspects of this disclosure.

DETAILED DESCRIPTION

Provided herein are methods and systems to compensate for banding causedby the interaction of Raster Output Scanner (ROS) variations with ahalftoned image. The interaction of ROS variations with a halftonedimage can cause an objectionable printed output density variation at aspatial frequency that is visually perceptible. The disclosed method andsystem provide the use of one or more fast scan beam delays operativelyassociated with a multibeam ROS as an actuator for a compensation schemeto reduce banding. Examples of a multibeam ROS include, but are notlimited to, a VCSEL (Vertical-Cavity Surface-Emitting Laser), andmultibeam laser diodes. The fast scan beam delays may be positive ornegative to delay or advance, respectively, to a fast scan beam relativeto one or more other beams or a nominal value. The provided actuator hasenough compensation authority to perform the compensation function.Tests demonstrate the compensation holds for a particular ROS overdifferent machines and does not significantly contribute to other imagequality defects. Benefits of the disclosed method include improving theimage quality of printed output and enabling ROS modules with certainerrors to function properly in a printing system.

As discussed in the background section, in any reprographic printingsystem, there are various sources of noise and errors that can result indensity variations in a printed output in the process direction, whichis also referred to as the slow scan direction. Periodic variations ofimage density in the process direction are typically known as banding.The classic method of eliminating banding, i.e., reduce it to animperceptible level, is to reduce/eliminate the various source of noiseand errors contributing to it.

In a reprographic printing system that uses a ROS to produce an aerialimage on a photoreceptor drum or belt for processing by the printingsystem, there are various errors that occur at relatively high spatialfrequencies. At these relatively high spatial frequencies, theperceptibility of density variations decreases significantly, sorelatively large errors at the high spatial frequencies are tolerable.However, in printing xerographic representations of continuous toneimages, halftones are used to convert the continuous tone images intobinary data suitable for printing with a xerographic system. Undercertain circumstances, the high spatial frequency ROS errors mix, i.e.,beat, with the halftone frequencies, resulting in highly visible lowerspatial frequency banding. Thus, if the errors at the high spatialfrequencies cannot be reduced enough to eliminate visibility of thelower spatial frequency beat frequency banding, then compensationtechniques need to be used to reduce the residual low frequency bandingto an unperceivable level.

As the required performance of reprographic printing systems increases,printing system specifications become tighter and tighter. Consequently,it is much more difficult to reduce the sources of errors contributingto halftone-interaction banding to meet these tighter system specs,hence the increased importance of banding compensation.

For example, a multibeam ROS associated with one xerographic printingsystem, with a product-intent 180 dpi halftone, produces a primaryinteraction/beat frequency of the associated halftone with a 32 beam ROSswath, which, in overwrite mode, moves ˜16 beams per swath, of 766 Hz(1.31 cycles/mm), and is very visible in prints.

Within-swath errors, such as (a) variations in beam power, (b)variations in the slow scan position of beams, and (c) variation in thefast scan position of beams, contribute to 766 Hz banding. In addition,swath to swath slow scan spacing errors also contribute to 766 Hzbanding. Any one of these sources of error can be used as an actuator toinject a known amount of error that compensates for the original error,via the concept of frequency-based vector/phasor addition.

In an exemplary multibeam laser diode array there is a named ASIC(Application-specific Integrated Circuit) that controls the operation ofthe laser source. Note that this source consists of 32 lasers and each“scanline” of the ROS consists of 32 beams scanned simultaneously.Included in the named ASIC is the ability to vary the timing of eachbeam individually in the fast scan direction. For purposes of thisdisclosure, and the exemplary embodiments disclosed herein, these arecalled “beam delays”, or in the alternative, “beam timing values”, andare normally used to ensure all beams “line up” vertically in a swath asthe swath is scanned in the fast scan direction. If one intentionallyadvances or delays a beam a small amount relative to the others, thisgenerates a known component of 766 Hz banding which can be used tocompensate for the vector resultant of the other sources of 766 Hzbanding. To simplify matters, modifications to beam delays are NVM(non-volatile memory)-settable.

With reference to FIG. 1, now is described an exemplary method ofcompensating for banding caused by an interaction, i.e., beating, of ahalftone pattern and process direction density variations associatedwith a ROS by adjusting variable beam timing values during a ROS setupprocess. It is to be understood that the exemplary method of FIG. 1 isdirected to a single ROS associated with a single halftone pattern, forexample, a black toner ROS. However, it is also to be understood thedescribed banding compensation method can be applied independently toeach of a plurality of ROSs where each ROS is associated with a distinctcolorant and halftone pattern. Importantly, the frequency of the bandingbeing compensated for, according to the exemplary method and systemprovided herein, is dependent on the ROS configuration and halftonestructure associated with the ROS. Specifically, as will be furtherdescribed below, the swath characteristics of the ROS, including beamspacing, as well as the structure of the associated halftone, i.e.,frequency, angle and/or spot growth pattern, effect the relatively lowfrequency beating which manifests as banding and is being compensatedfor according to this disclosure and the exemplary embodiments describedherein.

Initially, the EBC (Electronic Banding Compensation) of halftoneinteraction banding process 100 begins at step 105 by installing a ROS,the initial beam timing values and the associated halftone pattern.

Next, at step 110, the process marks an image of a test patternincluding the associated halftone pattern. For example, a test patternmay include 50% area coverage of the associated halftone patterncovering nearly a full media sheet or pitch associated with aphotoreceptor or intermediate transfer belt.

Next, at step 115, the process scans the marked test pattern.

Next, at step 120, the process analyzes the test pattern to determine afrequency and amplitude of banding present in the printed test patternwhich is attributable to an interaction of the ROS and the associatedhalftone.

Next, at step 125, the process determines if the banding amplitude iswithin the specifications of the printing system.

If the banding is within the specifications of the printing system,i.e., not perceptible under normal viewing conditions, no modificationof the ROS beam timing values, i.e., beam delays, is necessary and theprocess associates initial beam timing values with the ROS and halftoneat step 135.

If the banding is not within the specifications of the printing system,the process advances to step 130 where the ROS beam timing values aremodified, either by advancing or delaying one or more beams, relative toone or more other beams, to reduce the banding amplitude within thespecification of the printing system.

Finally, at step 135, the process stores the modified beam timing valuesin a memory and associates the modified beam timing values with theinstalled ROS and halftone pattern.

While the EBC process described with reference to FIG. 1 provides adescription of the basic processes involved in performing an exemplaryEBC process utilizing beam timing value modifications to delay oradvance a ROS beam relative to the other ROS/Swath beams, furtherdetails of the operation of the subject EBC, as well as a specificexample and results, and a general background of vector/phasor addition,are provided below.

Any banding compensation technique, where banding is defined as aparticular process-direction density variation at a particular spatialfrequency on a print, uses the concept of vector, or phasor, addition.

The EBC (Electronic Banding Compensation) process for 1×/2× banding asdescribed in U.S. patent application Ser. No. 13/313,533, filed Dec. 7,2011, now U.S. Publication No. 2013/0148172, published Jun. 13, 2013, byRobert Herloski et al. and entitled “Printing System, Raster OutputScanner, and Method with Electronic Banding Compensation UsingFacet-Dependent Smile Correction” and U.S. patent application Ser. No.13/334,251, filed Dec. 22, 2011, now U.S. Publication No. 2013/0163055,published Jun. 27, 2013, by Robert Herloski et al. and entitled “Processfor Creating Facet-Specific Electronic Banding Compensation Profiles forRaster Output Scanners” uses the concept of vector, or phasor, addition.This disclosure, and the exemplary embodiments included herein, alsouses the concept of vector, or phasor, addition for 766 Hz EBC.

A key “signature” for phasor addition is the appearance of asinusoidal-like variation in response as a function of phase.

Described now are two tests that were run on a ROS configured accordingto an exemplary embodiment of this disclosure. FIG. 2 shows a plot ofvarying the beam delay applied to a beam by 30 μm and FIG. 3 shows aplot of varying the intensity of a beam by 24%.

Both techniques produce a sinusoid-like variation in response and thebeam delay technique appears to have more dynamic range needed to getfull compensation. FIG. 4 is a repeat on another ROS:

From the figures it becomes apparent, if a delay is applied to beams 9or 17, a substantially full compensation of the 766 Hz dL* (imagebrightness variation) banding can be achieved.

FIG. 5 shows a plot of another example, showing a reduction in the FFT(Fast Fourier Transform) signature.

There are many potential variations for using beam delays to perform 766Hz EBC. For example, one beam alone can be delayed, or two beams can bedelayed, or all beams can be varied sinusoidally with an appropriatephase. A key concept here is to use whatever combination of beams isneeded, such that the Fourier transform of the total beam delays, e.g.,32 beams, produces an appropriate spatial frequency phasor thatcompensates for the initial errors.

One method of determining the appropriate beam delays to compensate forbanding at a particular frequency associated with a particular halftonestructure includes deterministic measurements where the bandingperformance of a ROS and associated halftone pattern is essentiallyquantified relative to a range of beam delay changes for a plurality ofROS beams and ultimately the beam delay values are selected based on themeasured performance levels and range of beam delay values to reduce thebeating of the ROS error frequency with the associate halftone patternto within the dL* specification of the printing system. As previouslydiscussed, the banding frequency will vary depending on the halftonestructure associated with a particular ROS design. In other words,differences in the frequency, angle and dot growth design between twohalftone patterns will produce different banding frequencies which arebeing compensated for according to the exemplary methods and systemsdescribed herein. Banding frequencies in the range of 5-3000 Hz are ofparticular significance, however, the disclosed method and system is notlimited to a specific banding frequency or range of banding frequencies.

Referring now to FIGS. 6-9, as discussed above, a process 100 is used togenerate electronic banding compensation beam delays according to thisdisclosure. Furthermore, a processing system using on-board processingelements 422 and sensors 460, or a process 100 using externalcomputational and analytical components can be used to implement theimage process including EBC as described herein. FIGS. 6-9 show anexemplary document processing system 400 (FIG. 6) and a ROS 500 thereof(FIGS. 7-9) in which stored banding correction beam delay values 506 areused to selectively delay or advance the timing of one or more modulatedlight outputs 522 to mitigate banding in a normal printing operation. Inaddition, the system 400, if equipped with a scanner or other sensors460, may implement automated banding compensation in accordance with theabove described processes. The illustrated document processing system400 is a multi-color printing system including a system controller 422with which one or more of the above described processing functions maybe implemented. In this system 400, included are multiple individualmarking devices 402, i.e. print engines, each including a ROS 500 asshown in FIGS. 7-9. Each ROS of each marking device 402 may be initiallysetup or thereafter adjusted for banding correction in accordance withthe method 100 above. The marking devices 402 individually transfertoner marking material from a photoreceptor drum (not shown) to anintermediate transfer belt (ITB) 404 traveling past the marking devices402. In this regard, the banding compensation techniques illustrated anddescribed herein can be employed in intermediate transfer belt (ITB)type systems as further described below and/or in non-ITB systems,wherein images are transferred directly from a photoreceptor drum orbelt directly to a print medium such as sheet paper without the use ofan ITB.

As shown in FIG. 7, in certain embodiments, each marking device 402includes a cylindrical drum photoreceptor 504 employed as anintermediate transfer substrate for subsequent transfer to theintermediate transfer belt 404 before final image transfer to a finalprintable medium 408, such as cut sheet paper. With reference to FIG. 6,the illustrated printing system 400 includes a transfer station 406downstream of the marking devices 402 to transfer marking material fromthe ITB 404 to an upper side of a final print medium 408 traveling alonga path P1 from a media supply. After the transfer of toner to the printmedium 408 at the transfer station 406, the final print medium 408 isadvanced to a fuser type affixing apparatus 410 along a path P1 wherethe transferred marking material is fused to the print medium 408. Inother embodiments, a single photoreceptor belt 404 is used with themarking devices 402 forming an image on the photoreceptor belt 404, andthe image developed on the belt is directly transferred to a printedmedium 408.

The system controller 422 performs various control functions and mayimplement digital front end (DFE) functionality for the system 400. Inaddition, the document processing system 400 may implement the abovedescribed techniques for adjusting the timing of the ROS beams tocompensate for banding. In this regard, the controller 422 may implementthe above described process 100 using the marking engines 402 and one ormore sensors 460. The controller 422 can be any suitable form ofhardware, processor-executed software and/or firmware, programmablelogic, or combinations thereof, whether unitary or implemented indistributed fashion in a plurality of processing components.

In a normal printing mode, the controller 422 receives incoming printjobs 418 and operates one or more of the marking devices 402 to transfermarking material onto the ITB 404 in accordance with image data of theprint job 418. In a banding compensation adjustment mode oralternatively during the normal printing mode, the controller 422operates in accordance with the above described process 100. Duringoperation of the marking devices 402, marking material (e.g., toner 451for the first device 402 in FIG. 6) is supplied to an internal drumphotoreceptor 504 (schematically shown in FIG. 7) via a ROS 500 of themarking device 402, and toner 451 is transferred to the ITB 404 with theassistance of a biased transfer roller (not shown) to attract oppositelycharged toner 451 from the drum 504 onto the ITB surface as the ITB 404passes through a nip between the drum 504 and the biased transferroller. The toner 451 ideally remains on the surface of the ITB 404after it passes through the nip for subsequent transfer and fusing tothe final print media 408 via the transfer device 406 and fuser 410 inFIG. 6. In the multicolor example of FIG. 6, each xerographic markingdevice 402 is operable under control of the controller 422 to transfertoner 451-454 of a corresponding color (e.g., cyan (C), magenta (M),yellow (Y), black (K)) to the transfer belt 404. The system 400 alsoincludes one or more sensors 460 internal to the marking stations 402and/or external thereto, for instance, to measure one or more markingmaterial transfer characteristics such as toner density relative to theintermediate transfer belt 404 or other photoreceptor or with respect toa final printed medium 408, and corresponding feedback signals or valuesare provided to the controller 422. Notably, as previously discussed,toner density variations measured for a printed test pattern are used toset up the ROS, i.e. beam delay values, to perform EBC for a measuredbanding frequency associated with the beating of high frequencies.

As seen in FIGS. 7-9, the exemplary xerographic stations 402 eachinclude a multi-beam ROS 500 which generates latent images along acircuitous length of a drum type photoreceptor 504 (shown in partialsection view with the process direction into the page in FIG. 7) using aplurality of beams 522. A ROS controller 502 provides one or morecontrol signals or values to a driver 512 and a ROS clock 501, and astream of image data is provided from the controller 502 to the driver512 associated with 32 laser-type light sources 514, for instance,arranged as a laser emitter array of eight groups of four lasers in oneembodiment. The ROS controller 502 also operates the ROS clock 501,which in turn provides a clock output to the driver 512 and to a motorpolygon assembly (MPA) that includes a polygon motor speed control 528 aand the rotating polygon 528 with a plurality of reflective (e.g.,mirrored) outer surfaces or facets 526 (eight facets 526 are shown forillustration in the example of FIG. 7, but other embodiments may havemore or fewer facets 526).

In operation, a stream of image data is provided to the driver 512associated with a single color portion of a panel image in the printer400, and the driver 512 modulates the lasers 514 to produce a pluralityof modulated light outputs or beams 522 in conformance with the inputimage data. The laser beam light output 522 passes into conditioningoptics 524 and then illuminates a facet 526 of the rotating polygon 528.The light beams 522 are reflected from the polygon facet 526 throughimaging optics 530 to form corresponding spots on the photosensitiveimage plane portion of the passing photoreceptor 504 drum. Rotation ofthe facet 526 causes the spots to be swept or scanned across the imageplane in the cross-process or fast scan direction FS to form asuccession of scan lines generally perpendicular to a “slow scan” orprocess direction PD along which the photoreceptor 504 travels. In themulti-beam arrangement of the ROS 500, 32 such scan lines are createdconcurrently as a group or swath with the image data provided to theindividual lasers 514 being interleaved accordingly. Successive rotatingfacets 526 of the polygon 528 form successive sets or swaths of 32 scanlines that are offset from each other as the photoreceptor 504 travelsin the process direction. In this regard, each facet 526 may scan 32scan lines, but the photoreceptor 504 may move such that the top 16 scanlines from the next facet 526 can overlap the bottom 16 scan lines fromthe previous facet 526 in an interleaved or overlapped fashion. In thisregard, the disclosed methods of EDC can be used in systems in whichscan lines are overwritten (overlapped) with or without interleaving,and/or in systems that employ interleaving with scan lines from asubsequent swath written in between scan lines from a previous swath, orcombinations or variations thereof.

Within each set of 32 scan lines, moreover, the laser emitter array 514provides mechanical spacing of the individual light outputs 522 suchthat the spacing of adjacent scan lines is ideally uniform. Each suchscan line in this example consists of a row of pixels produced bymodulation of the corresponding laser beam 522 according to thecorresponding image data as the laser spots scan across an image plane,where individual spots are either illuminated or not at various pointsas the beams scan across the scan lines so as to selectively illuminateor refrain from illuminating individual locations on the photoreceptor504 according to the input image data. In this way a latent image iscreated by selectively discharging the areas of the photoreceptor 504which are to receive a toner image. Exposed (drawn) portions of theimage to be printed move on to a toner deposition station (not shown)where toner adheres to the drawn/discharged portions of the image. Theexposed portions of the image with adherent toner then pass to atransfer station with a biased transfer roller (BTR, not shown) fortransfer of the toner image to the intermediate transfer belt (ITB 404in FIG. 6 above).

As seen in FIGS. 7-9, moreover, the ROS driver 512 selectively employsbanding correction or beam timing control 506 under direction of the ROScontroller 502 to vary the timing of the light outputs 522 provided bythe light source or sources 514 during scanning in order to mitigatebanding in the final print media 408. The MPA polygon 528 then directsthe modulated light outputs 522 toward the photoreceptor medium 504along the fast scan direction to generate an image on at least a portionof the medium 504. As seen in FIG. 7, moreover, rotation of the currentfacet 526 of the polygon 528 scans the one or more modulated lightoutputs 522 directly or indirectly to the photoreceptor 504 along thefast scan direction FS, where one or more optical components may liebetween the polygon facet 526 and the photoreceptor 504, where onesimplified example (lens 530) is illustrated in FIG. 7. In operation,the controller 502 of the ROS 500 (FIGS. 7-9) causes the driver 512 toselectively vary the timing of the light output(s) 522 provided by thelight source(s) 514 during scanning so as to mitigate banding.

In certain embodiments, the ROS 500 includes an MPA encoder 508 whichprovides an output to the ROS controller 502, which can be any signal orvalue that indicates the identity of a reflective facet 526 of therotating polygon 528 that is currently scanning light output(s) 522.

As seen in FIG. 8, in certain embodiments that use multiple lightsources 514 (e.g., an array of 32 lasers 514 in the illustratedexample), the controller 502 may cause the driver 512 to selectivelyvary the beam timing of all the modulated light outputs 522 provided bythe light sources 514. In one possible implementation, the ROS 500 mayinclude programmable logic, such as an application specific integratedcircuit (ASIC) that controls the operation of the laser source(s) 514.

FIG. 9 shows another example using multiple laser light sources 514(e.g., 32 in the illustrated implementation), in which more than onebanding correction beam delays are used. The ROS controller 502 causesthe driver 512 to selectively vary the beam delay values of thecorresponding laser light sources 514 according to the corresponding oneof the 32 beam delay values. In one possible implementation, the ROS 500may include an ASIC or other logic providing the capability to modify 32individual beam values, wherein the controller 502 can utilize suchlogic to control the beam timing.

The above embodiments thus allow the process direction banding affectsto be corrected on a scanline-by-scanline basis and/or on aswath-by-swath basis (electronic banding correction or compensation),thereby facilitating control over measurable MPA harmonic banding in agiven document processing system 400, including variation of amplitudeand phase in the process direction, wherein the ROS controller 502 canemploy a beam timing control function, varying beam timing and phase inthe cross-process/fast scan direction, which will compensate for MPAharmonic banding at all fast scan locations between the start of scan(SOS) and the end of scan (EOS) locations.

Some portions of the detailed description herein are presented in termsof algorithms and symbolic representations of operations on data bitsperformed by conventional computer components, including a centralprocessing unit (CPU), memory storage devices for the CPU, and connecteddisplay devices. These algorithmic descriptions and representations arethe means used by those skilled in the data processing arts to mosteffectively convey the substance of their work to others skilled in theart. An algorithm is generally perceived as a self-consistent sequenceof steps leading to a desired result. The steps are those requiringphysical manipulations of physical quantities. Usually, though notnecessarily, these quantities take the form of electrical or magneticsignals capable of being stored, transferred, combined, compared, andotherwise manipulated. It has proven convenient at times, principallyfor reasons of common usage, to refer to these signals as bits, values,elements, symbols, characters, terms, numbers, or the like.

It should be understood, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise, as apparent from the discussion herein,it is appreciated that throughout the description, discussions utilizingterms such as “processing” or “computing” or “calculating” or“determining” or “displaying” or the like, refer to the action andprocesses of a computer system, or similar electronic computing device,that manipulates and transforms data represented as physical(electronic) quantities within the computer system's registers andmemories into other data similarly represented as physical quantitieswithin the computer system memories or registers or other suchinformation storage, transmission or display devices.

The exemplary embodiment also relates to an apparatus for performing theoperations discussed herein. This apparatus may be specially constructedfor the required purposes, or it may comprise a general-purpose computerselectively activated or reconfigured by a computer program stored inthe computer. Such a computer program may be stored in a computerreadable storage medium, such as, but is not limited to, any type ofdisk including floppy disks, optical disks, CD-ROMs, andmagnetic-optical disks, read-only memories (ROMs), random accessmemories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any typeof media suitable for storing electronic instructions, and each coupledto a computer system bus.

The algorithms and displays presented herein are not inherently relatedto any particular computer or other apparatus. Various general-purposesystems may be used with programs in accordance with the teachingsherein, or it may prove convenient to construct more specializedapparatus to perform the methods described herein. The structure for avariety of these systems is apparent from the description above. Inaddition, the exemplary embodiment is not described with reference toany particular programming language. It will be appreciated that avariety of programming languages may be used to implement the teachingsof the exemplary embodiment as described herein.

A machine-readable medium includes any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputer). For instance, a machine-readable medium includes read onlymemory (“ROM”); random access memory (“RAM”); magnetic disk storagemedia; optical storage media; flash memory devices; and electrical,optical, acoustical or other form of propagated signals (e.g., carrierwaves, infrared signals, digital signals, etc.), just to mention a fewexamples.

The methods illustrated throughout the specification, may be implementedin a computer program product that may be executed on a computer. Thecomputer program product may comprise a non-transitory computer-readablerecording medium on which a control program is recorded, such as a disk,hard drive, or the like. Common forms of non-transitorycomputer-readable media include, for example, floppy disks, flexibledisks, hard disks, magnetic tape, or any other magnetic storage medium,CD-ROM, DVD, or any other optical medium, a RAM, a PROM, an EPROM, aFLASH-EPROM, or other memory chip or cartridge, or any other tangiblemedium from which a computer can read and use.

Alternatively, the method may be implemented in transitory media, suchas a transmittable carrier wave in which the control program is embodiedas a data signal using transmission media, such as acoustic or lightwaves, such as those generated during radio wave and infrared datacommunications, and the like.

It will be appreciated that variants of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be combined intomany other different systems or applications. Various presentlyunforeseen or unanticipated alternatives, modifications, variations orimprovements therein may be subsequently made by those skilled in theart which are also intended to be encompassed by the following claims.

What is claimed is:
 1. A raster output scanner (ROS) for generating an image on a portion of a light-receiving medium traveling along a process path past the ROS, the ROS comprising: a plurality of light sources operative to generate a plurality of light outputs; a driver operatively associated with the plurality of light sources and operative to selectively actuate the plurality of light sources individually according to corresponding scan line image data to produce a corresponding plurality of modulated light outputs, the driver operatively associated with a memory storing a plurality of beam timing values corresponding to each of the plurality of light sources, the beam timing values set to compensate for banding caused by an interaction with a halftone pattern and process direction density variations associated with the ROS, each beam timing value associated with a cross-process direction delay or advancement of a respective light source, the driver configured to selectively actuate the plurality of light sources according to the corresponding beam timing values based on the scan line image data to produce the plurality of modulated light outputs; and a scanner that directs the modulated light outputs from the light sources toward a light receiving medium along the cross-process direction to generate an image on at least a portion of the light receiving medium, wherein the beam timing values associated with the halftone pattern provides one of reducing and minimizing the banding caused by the interaction with the halftone pattern.
 2. The ROS of claim 1, wherein the plurality of light sources is a laser emitter array including a plurality of lasers individually operative to generate light outputs, the light-receiving medium is a photoreceptor associated with a printing system, and the scanner is operative to direct the light outputs from the laser emitter array toward the photoreceptor along the cross-process direction to generate a latent image on at least a portion of the photoreceptor.
 3. The ROS of claim 2, wherein the scanner includes a polygon mirror with a plurality of reflective surfaces and a polygon motor speed control operative to control rotation of the polygon mirror, where reflection of the modulated light outputs from the laser emitter array off at least one reflective surface of the polygon mirror directs the modulated light outputs toward the photoreceptor.
 4. The ROS of claim 1, wherein the scanner includes a polygon mirror with a plurality of reflective surfaces and a polygon motor speed control operative to control rotation of the polygon mirror, where reflection of the modulated light outputs from the light sources off at least one reflective surface of the polygon mirror directs the modulated light outputs toward the light-receiving medium.
 5. The ROS of claim 1, wherein the beam timing values are associated with compensating for a periodic banding structure including a spatial frequency in a range of 5-3000 Hz.
 6. The ROS of claim 1, wherein the beam delay values are associated with a first set of beam timing values associated with one of a plurality of halftone patterns, and the driver is configured to use a plurality of sets of beam timing values, each set of beam timing values corresponding to one of the plurality of halftone patterns.
 7. The ROS of claim 1, wherein the beam timing values associated with the halftone pattern are determined by varying one or more beam timing values until the banding caused by the interaction with the halftone pattern is one of reduced and minimized.
 8. (canceled)
 9. A document processing system comprising: at least one marking device operative to transfer marking material onto an intermediate medium; a transfer station positioned proximate a travel path of the intermediate medium and operative to transfer the marking material from the intermediate medium to a printable media; and a raster output scanner (ROS) operatively associated with the at least one marking device to generate an image on a portion of a light-receiving medium, the ROS including: a plurality of light sources operative to generate a plurality of light outputs; a driver operatively associated with the plurality of light sources and operative to selectively actuate the plurality of light sources individually according to corresponding scan line image data to produce a corresponding plurality of modulated light outputs, the driver operatively associated with a memory storing a plurality of beam timing values corresponding to each of the plurality of light sources, the beam timing values set to compensate for banding caused by an interaction with a halftone pattern and process direction density variations associated with the ROS, each beam timing values associated with a cross-process direction delay or advancement of a respective light source, the driver configured to selectively actuate the plurality of light sources according to the corresponding beam timing values based on the scan line image data to produce the plurality of modulated light outputs; and a scanner that directs the modulated light outputs from the light sources toward a light receiving medium along the cross-process direction to generate an image on at least a portion of the light receiving medium, wherein the beam timing values associated with the halftone pattern provides one of reducing and minimizing the banding caused by the interaction with the halftone pattern.
 10. The document processing system according to claim 9, wherein the plurality of light sources is a laser emitter array including a plurality of lasers individually operative to generate light outputs, the light-receiving medium is a photoreceptor associated with a printing system, and the scanner is operative to direct the light outputs from the laser emitter array toward the photoreceptor along the cross-process direction to generate a latent image on at least a portion of the photoreceptor.
 11. The document processing system according to claim 10, wherein the scanner includes a polygon mirror with a plurality of reflective surfaces and a polygon motor speed control operative to control rotation of the polygon mirror, where reflection of the modulated light outputs from the laser emitter array off at least one reflective surface of the polygon mirror directs the modulated light outputs toward the photoreceptor.
 12. The document processing system according to claim 9, wherein the scanner includes a polygon mirror with a plurality of reflective surfaces and a polygon motor speed control operative to control rotation of the polygon mirror, where reflection of the modulated light outputs from the light sources off at least one reflective surface of the polygon mirror directs the modulated light outputs toward the light-receiving medium.
 13. The document processing system according to claim 9, wherein the beam timing values are associated with compensating for a periodic banding structure including a spatial frequency in a range of 5-3000 Hz.
 14. The document processing system according to claim 9, wherein the beam timing values are associated with a first set of beam timing values associated with one of a plurality of halftone patterns, and the driver is configured to use a plurality of sets of beam timing values, each set of beam timing values corresponding to one of the plurality of halftone patterns.
 15. The document processing system according to claim 9, wherein the beam timing values associated with the halftone pattern are determined by varying one or more beam timing values until the banding caused by the interaction with the halftone pattern is one of reduced and minimized.
 16. (canceled)
 17. A method of manufacturing or adjusting a document printing system, the method comprising: operatively coupling a raster output scanner (ROS) with a marking station in a document processing system; determining a plurality of beam timing values corresponding to individual ones of a plurality of light sources of the ROS so as to compensate for banding caused by an interaction with a halftone pattern and process direction density variations associated with the document printing system; and storing the plurality of beam timing values in a memory associated with the ROS, wherein the beam timing values associated with the halftone pattern provides one of reducing and minimizing the banding caused by the interaction with the halftone pattern.
 18. The method of manufacturing or adjusting a document printing system according to claim 17, comprising: determining a plurality of sets of beam timing values corresponding to individual ones of a plurality of light sources of the ROS, each set of beam timing values corresponding to one of a plurality of halftones; and storing the plurality of sets of beam timing values in a memory associated with the ROS.
 19. The method of manufacturing or adjusting a document printing system according to claim 17, wherein the beam timing values associated with the halftone pattern are determined by varying one or more beam timing values until the banding caused by the interaction with the halftone pattern is one of reduced and minimized.
 20. (canceled) 